As optical lithography systems are designed to print smaller feature sizes, their operational wavelengths are being reduced, and their numerical apertures (NAs) are being increased. Current conventional lithographic production utilizes 248-nm (nanometer) wavelengths, and numerical apertures from 0.6 to as high as about 0.75. Small-field exposure systems in development use NAs as high as 0.9.
FIG. 1 is a schematic illustrating parameters used to specify NA. Lens system 100 is illustrated as a single lens element. However, NA can be specified for an arbitrarily complicated lens system. For example, system 100 may be a lithographic projection system.
NA is defined by the equation:NA=n sin(θ);where θ is the semi-angle (measured from optical axis 120) of a cone of rays 130 forming an image on a surface 150; and n is the index of refraction of region 110 on the image side of system 100 (i.e., the side of system 100 on which the semi-angle θ is measured). For a typical lens system, region 110 is filled with air or nitrogen, both having indices of refraction that are substantially equal to 1.
In the case of lithographic systems, the image formed by lens system 100 is typically projected into a thin layer of photosensitive resist 152 (also referred to as a photoresist), which is subsequently processed to make a circuit design of a semiconductor device. As rays 130 enter resist 152, they are refracted. At wavelengths of 193-nm and 157-nm, the index of refraction nr of photoresist 152 is typically approximately 1.5, and at longer wavelengths, such as the 365-nm commonly used for semiconductor lithography, the index of refraction nr of photoresist 152 is often as high as about 1.8.
According to Snell's Law, upon transmission from a lower index material (e.g., air) into a higher index material (e.g., photoresist 152), the incident rays are brought closer to the optical axis (i.e., θr is smaller than θ). For a ray at the limit of the lens NA, the angle of the ray in the resist is given by the following equation:sin(θr)=sin(θ)/nr=NA/nr;where θr=the angle (from the surface normal) of the ray in the resist; and θ=the angle of the ray, in the medium above the resist 152 (e.g., air).
The above equation illustrates that for lens systems having small NAs, and a high-index resist 152, rays are relatively close to optical axis 120. Accordingly, for such systems it has generally been assumed that rays are sufficiently close to optical axis 120 that one can neglect the vector nature of the incident light. However, as NAs are increased and nr is decreased, ray angles depart substantially from normal incidence, and the vector nature of light affects the image contrast. The nature of the reduction in contrast is described in greater detail below.
FIG. 2 is a schematic diagram of a simple, but representative, conventional lithographic system 200. Imaging system 200 includes, for example, a so-called chromeless phase-shifting mask 210, having a phase-shift grating structure 212 (i.e., an array of closely spaced etched features having a height which delays light passing through the thicker regions by one-half of the operational wavelength relative to the light passing through the thinner regions). In system 200, light 222 from an illumination source 220 is used to expose grating structure 212, and grating structure 212 is imaged onto a surface 250 by lens system 202.
When grating structure 212 is illuminated with spatially coherent light at near-normal incidence to mask 210, the phase shift grating structure 212 on mask 210 results in two diffracted beams 230a and 230b. FIG. 2 is illustrated using a coordinate system where grating structure 212 is aligned in an X-Y plane, an optical axis 240 is aligned with the Z-axis, and grating elements in grating structure 212 are aligned with the X-axis. Accordingly, diffracted beams 230a, 230b lie in a Y-Z plane of incidence. While not shown in FIG. 2, is should be appreciated that a grating structure having grating elements aligned parallel to the Y-axis and illuminated with light 222 would produce diffracted beams lying in a X-Z plane of incidence.
A “plane of incidence” of a ray is defined as the plane containing the ray, and the normal vector (e.g., optical axis 240) of the incident surface (e.g., surface 250). For the simple case of a mask having all grating elements aligned in the direction of one axis, as illustrated in FIG. 2, all rays (e.g., 230a and 230b) lie in the same plane of incidence. However, rays emanating from a mask having a more complex pattern (i.e., a mask having features aligned in multiple directions) arrive at surface 250 in multiple planes of incidence.
As one of ordinary skill would understand, as the density of the grating elements of the grating structure 212 increases (i.e., the grating pitch of grating structure 212 decreases), the angles θr that diffracted beams 230a and 230b make with optical axis 240 increase. In FIG. 2, grating structure 212 is illustrated having a pitch that causes diffracted beams 230a and 230b to emerge at angles θr just within the NA of the imaging system. Such a grating structure pushes optical system 200 to near its theoretically limiting performance.
When the beams 230a and 230b depart substantially from normal incidence with surface 250 (i.e., NA is relatively large), the vector nature of the incident light affects the image contrast at surface 250. Specifically, it has been observed that as the NA is increased, the portion of light incident on surface 250 that is s-polarized (i.e., the portion of the light having an electric field vector Es perpendicular to the plane of incidence) interferes to form a grating image of high contrast for all angles of incidence θ, while light that is p-polarized (i.e., the portion of the light having an electric field vector Ep parallel to the plane of incidence) has reduced (or even no) image contrast. The degree of reduction in contrast is a function of the angle of incidence θ.
One of ordinary skill will understand that components of light in a beam of light (e.g., beam 230a or 230b) are determined to be s-polarized or p-polarized based, in part, on the light's direction of impingement on surface 250. Because the direction of a ray is determined by the angle of diffraction caused by mask 210, a ray impinging on mask 210 may become an s-polarized or p-polarized ray depending on whether the feature diffracting the ray is aligned with the x-axis or the y-axis.
In general, the interference of the electric fields from the respective s-polarized and p-polarized beams 230a and 230b gives rise to the image pattern at surface 250, and determines the contrast of the image at surface 250. For s-polarized light, the electric field vectors Es of beams 230a and 230b are parallel, since they are both normal to the plane of incidence (perpendicular to the plane of FIG. 2), thus ensuring that there are electric field vector components to interfere. By contrast, for p-polarized light, when beams 230a and 230b arrive at near normal incidence to surface 250, (e.g., due to low NA or high index resist) the electric field vectors Ep of beams 230a and 230b are nearly parallel, and interfere effectively. However, in a case in which beams 230a and 230b are at a 90-degree angle to each other, as measured in the plane of incidence (i.e., each of the beams 230a and 230b impinges on surface 250 at 45 degrees relative to the optical axis 240), the electric field vectors Ep are normal to one another, and therefore there are no common electric field vector components to interfere. Thus, there is essentially no image contrast in a two-beam exposure for the p-polarization when the rays 230a and 230b arrive at 90 degrees to each other.
In view of the foregoing, it may be appreciated that for the extreme case of two rays at 90 degrees to each other, one would have virtually 100% image contrast for the s-polarization and 0% image contrast for the p-polarization. If one used unpolarized incident light (i.e., light having a randomly varying polarization over an exposure duration) incident light on the mask, one would obtain a contrast of only 50%.
One prior art solution to the problem of reduced contrast in a lithographic system involves dividing mask data (i.e., the grating pattern of mask 210 of FIG. 2) into two masks, each mask having features oriented substantially along a single axis (i.e., the X-axis, or the Y-axis). In such a system, each mask is exposed using light having a single polarization; to achieve improved contrast, the polarization is selected such that only s-polarized light is used to form the resulting image during each exposure. Drawbacks of such a system include the expense of producing two masks to produce a single image, and the presence of overlay errors that result in reductions in image quality. The overlay errors arise from the need to align the outputs formed by the two exposures necessary to obtain a single image containing information from both masks.